I hope this is what you need. When I'm typing an email in Gmail or a text and type a word that isn't in the dictionary, I tap on the check mark on the left side of the screen just above the keyboard. That makes the word "stick" (vs auto correcting it) and it will be recognized from now on. Discovered this on m Galaxy Note 2014. Hope it helped.
how long is temporarily? the three words that I used and saved by tapping on the check mark, within the last week, are still in the personal dictionary. I've had my note 10.1 since December and haven't had problems with added words disappearing. sorry I couldn't help.